In 1923 G. N. Lewis suggested another way of looking at the
reaction between H+ and OH- ions. In the
Brnsted model, the OH- ion is the active species in
this reaction
it accepts an H+ ion to form a covalent bond. In the
Lewis model, the H+ ion is the active speciesit
accepts a pair of electrons from the OH- ion to form a
covalent bond.

In the Lewis theory of acid-base reactions, bases donate pairs
of electrons and acids accept pairs of electrons. A Lewis acid
is therefore any substance, such as the H+ ion, that
can accept a pair of nonbonding electrons. In other words, a
Lewis acid is an electron-pair acceptor. A Lewis base
is any substance, such as the OH- ion, that can donate
a pair of nonbonding electrons. A Lewis base is therefore an electron-pair
donor.

One advantage of the Lewis theory is the way it complements
the model of oxidation-reduction reactions. Oxidation-reduction
reactions involve a transfer of electrons from one atom to
another, with a net change in the oxidation number of one or more
atoms.

The Lewis theory suggests that acids react with bases to share
a pair of electrons, with no change in the oxidation numbers of
any atoms. Many chemical reactions can be sorted into one or the
other of these classes. Either electrons are transferred from one
atom to another, or the atoms come together to share a pair of
electrons.

The principal advantage of the Lewis theory is the way it
expands the number of acids and therefore the number of acid-base
reactions. In the Lewis theory, an acid is any ion or molecule
that can accept a pair of nonbonding valence electrons. In the
preceding section, we concluded that Al3+ ions form
bonds to six water molecules to give a complex ion.

Al3+(aq) + 6 H2O(l)
Al(H2O)63+(aq)

This is an example of a Lewis acid-base reaction. The Lewis
structure of water suggests that this molecule has nonbonding
pairs of valence electrons and can therefore act as a Lewis base.
The electron configuration of the Al3+ ion suggests
that this ion has empty 3s, 3p, and 3d
orbitals that can be used to hold pairs of nonbonding electrons
donated by neighboring water molecules.

Al3+ = [Ne] 3s0 3p0
3d0

Thus, the Al(H2O)63+ ion is
formed when an Al3+ ion acting as a Lewis acid picks
up six pairs of electrons from neighboring water molecules acting
as Lewis bases to give an acid-base complex, or complex
ion.

The Lewis acid-base theroy explains why BF3 reacts
with ammonia. BF3 is a trigonal-planar molecule
because electrons can be found in only three places in the
valence shell of the boron atom. As a result, the boron atom is sp2
hybridized, which leaves an empty 2pz
orbital on the boron atom. BF3 can therefore act as an
electron-pair acceptor, or Lewis acid. It can use the empty 2pz
orbital to pick up a pair of nonbonding electrons from a Lewis
base to form a covalent bond. BF3 therefore reacts
with Lewis bases such as NH3 to form acid-base
complexes in which all of the atoms have a filled shell of
valence electrons, as shown in the figure below.

The Lewis acid-base theory can also be used to explain why
nonmetal oxides such as CO2 dissolve in water to form
acids, such as carbonic acid H2CO3.

CO2(g) + H2O(l)
H2CO3(aq)

In the course of this reaction, the water molecule acts as an
electron-pair donor, or Lewis base. The electron-pair acceptor is
the carbon atom in CO2. When the carbon atom picks up
a pair of electrons from the water molecule, it no longer needs
to form double bonds with both of the other oxygen atoms as shown
in the figure below

One of the oxygen atoms in the
intermediate formed when water is added to CO2 carries
a positive charge; another carries a negative charge. After an H+
ion has been transferred from one of these oxygen atoms to the
other, all of the oxygen atoms in the compound are electrically
neutral. The net result of the reaction between CO2
and water is therefore carbonic acid, H2CO3.

Practice Problem 9:

Predict
whether the following ions or molecules can act as either
a Lewis acid or a Lewis base.